Two Undergrad MSci plant pathologists from the University of Nottingham

Plasma Irradiation for Pest Management

Today I will be discussing a paper that I read recently by Ochi et al. which was published in Plant Pathology in January 2017[2]. It discussed a possible alternative to chemical pesticides in the form of plasma irradiation. In a world where people are increasingly concerned about the use of chemicals on their food due to increased awareness, where multiple pathogens are showing resistance to chemical treatment and others are being banned due to possible effects on pollinators[3], this topic is both important and interesting.

Bakanae disease and seedling blight (figure 1) are seedborne diseases which infect rice, a staple food for much of the world (figure 2). They are caused by Fusariumfujikuroi and Burkholderia plantarii respectively. Bakanae has caused yield losses as high as 94.5% and is becoming an increasing threat to rice production. Approximately 800 million tonnes will need to be produced in 2025 to meet the growing population[4] hence why research like this is so important in order to find effective pest control mechanisms.

Figure 2 Rice consumption 2015/16 in thousand tonnes[5]Seedborne diseases need to be controlled in the early nursery stage of rice production. The usual method of disinfecting seeds is to put them in hot sterile water but sometimes pathogens survive this. With increased chemical resistance and incomplete water sterilisation, this paper looked at how plasma irradiation might be used.

What is plasma?

Plasma, along with solid, liquid and gas is one of the states of matter. It is a cloud of electrons with positive and negative charges. High-temperature plasma is found in stars but low-temperature plasma has the same pressure as the normal atmosphere. Reactive oxygen species (ROS), chemically active molecules containing oxygen, are produced in plasma and have antimicrobial activity so may be used to kill pathogens in seeds. ROS may damage cells so the key in plasma treatment is to treat for the right amount of time so to not affect the germinating properties of the seed.

Seed preparation and treating with plasma irradiation

The two microorganisms being studied were grown on agar and were used to inoculate rice seeds by soaking them in spore suspension. The seeds were then germinated by incubating at 30°C and grown for one month. B. plantarii was also inoculated via a second method, by spraying it on the flowers of already grown plants. Seeds were harvested from these infected plants. The apparatus in figure 3 was used to irradiate the mature seeds with plasma. Treatment was for 10 minutes and the seeds were rotated every 2 minutes by shaking the beaker.

The paper found that the seeds treated with plasma showed the same germination success as the controls which were not (figure 4a). This showed that the plasma treatment did not damage the seeds in a way that affected this process.

Disease was measured on a disease severity index, ranking it from 0-4 with 0 being healthy plants and 4 being severely infected or dead. There was an average reduction of 7.8% in disease symptoms (figure 4b-d) and 18.1% in the disease severity index (figure 4e-g) when comparing treated seeds to the controls.

Figure 4 a) Successful and similar seedling growth in both treated and control seeds, b-d) Difference in symptoms shown between treated and control in three repeats, e-g) Disease severity in three repeats using the disease severity index[2]Some seeds were immersed in water and then treated with plasma and others were only treated with plasma. The paper found that the seeds immersed in 50°C water and then treated with plasma had undetectable levels of fungal cells compared to the untreated seeds. However, the immersion and plasma treatment also lead to the production of hydrogen peroxide (H2O2), shown as a dark brown staining in the embryo tissues which can be seen in figure 5.

Overall, it was found that plasma treatment was most effective when combined with immersion in SDW and that it did not affect the germination and growth of the seedlings. It significantly reduced the growth of Bakanae and seedling blight.

More work needs to be carried out to understand the mechanism of how plasma treatment causes this effect and whether it is applicable to other seed-borne diseases. Different treatment times may be required depending on the disease and seed size. They speculated on the mechanism with comments on how due to the presence of H2O2, it is possible that hydrogen peroxide is mediating the production of reactive oxygen species capable of killing fungal cells. Another possibility is that the treatment is triggering the plant’s immune system so that the plant kills the fungal cells which could be further investigated through analysis of gene expression (like in my own project which you can read about here).

For now, it is safe to say that it is an effective method for integrated pest control and worth further research to bring it to a commercial level where it might be used as an alternative to chemical treatment.

Conclusion about the paper

I found this paper an interesting read as it is focusing on a similar topic area as my own project. It was mostly well written although the methods section was difficult to understand in parts due to the amount of detail and the number of different processes that were carried out. I am sure that to someone used to dealing with the two microorganisms it would have made more sense. There were clear figures which made it easier to understand how the plasma irradiation was setup and also the results. The paper also left a clear idea of where the work may be taken in the future with the understanding of the ROS mechanism, which was a good way to end.

You can read the original article here and find background reading that might be of interest below.